2-Methoxyestradiol (2-MeO-E2) is a nonpolar endogenous estrogen metabolite formed by metabolic O-methylation of 2-hydroxyestradiol (the most abundant hydroxyestrogen metabolite in humans). 2-MeO-E2 has anti-proliferative, apoptotic, and antiangiogenic actions at nonphysiological high concentrations. This grant application has two overall goals: (i) to study the molecular mechanism(s) of 2-MeO-E2's action by searching for its specific cellular receptor in human breast cancer cell lines. We will isolate a specific, high-affinity cellular receptor for 2-MeO-E2 present in human breast cancer cells, and we will determine its protein and DNA sequences (described under Specific Aim 1). (ii) To determine whether the endogenously-formed or exogenously-administered 2-MeO-E2 (at physiologically-relevant concentrations) has chemoprotective effects against estrogen-induced mammary tumor formation in a commonly-used animal model. We will determine the potency and efficacy of the exogenously-administered 2-MeO-E2 for protection against estradiol-induced mammary tumorigenesis in female ACI rats, and we will also evaluate the mammary cancer-protective effects of the endogenously-formed 2-MeO-E2 by determining whether chronic administration of entacapone (a selective COMT inhibitor) alters estradiol-induced mammary carcinogenesis in the female ACI rats. These studies are described under Specific Aims 2, 3, and 4. Our proposed studies are expected to advance our knowledge on the mammary cancer-protective effects of 2- MeO-E2, a nonpolar endogenous estrogen metabolite formed in large amounts in humans. This knowledge will form the basis for future development of new approaches to the prevention of human mammary cancer by administration of "physiological doses" of 2-MeO-E2 (or its synthetic analogs), or by using agents that can alter the metabolic formation and/or disposition of endogenous 2-MeO-E2 in beneficial ways. The results will also provide novel mechanistic understanding for the biological actions of 2-MeO-E2.